Plastics are used in large amount in wide industrial fields because of excellent properties such as easy shaping, lightweight, cheapness, durability and the like. However, due to durability, even if used plastics are discarded into the natural world, these are not decomposed to cause an environmental problem, in some cases. Since used plastics are not permitted to be discarded into the natural world, they should be subjected to a burning treatment and the like after use, however, due to large heat generation in burning, there is a possibility of injuring a combustion furnace in burning, and dioxin is generated by burning, in some cases. Based on such facts, there is desired a biodegradable plastic which can be recycled and which is decomposed by a microorganism and the like when discarded into the natural world after use. Particularly, from the standpoint of reduction of production energy and decrease in discharge amount of carbon dioxide, a material-recyclable biodegradable plastic is more desired than a thermal-recyclable biodegradable plastic.
However, conventional biodegradable plastics are insufficient in properties such as heat resistance and the like as compared with general plastics, in some cases. Therefore, for the purpose of improving the properties of a biodegradable plastic, such as heat resistance and the like, Japanese Patent Application Laid-Open (JP-A) No. 6-192375 suggests a technology in which polycaprolactone is cross-linked with an isocyanate, and the heat resistance of a biodegradable plastic is improved by introducing a cross-linked structure of a covalent bond.
In the above-mentioned conventional technology, the heat resistance and the like of a biodegradable plastic are improved by a cross-linked structure, however, there are a possibility of decrease in flowability in heat melting, a possibility of insufficient moldability, and a possibility of decrease in biodegradability. Particularly, in the case of a highly cross-linked biodegradable plastic, when this is once molded, it behaves as if a thermosetting resin, and even if this is to be recovered and recycled, sufficient heat melting is not attained in second and later moldings, leading to difficult recycling, in some cases.
For the purpose of improving a recycling property, there is a suggestion on introduction of a thermo-reversible cross-linked structure with covalent bond into a plastic. First, as examples of a thermo-reversible reaction based on covalent bond, Engle et al., J. Macromol. Sci. Re. Macromol. Chem. Phys., vol. 33, no. C3, pp. 239 to 257, 1993 describes a Diels-Alder reaction, nitroso dimerization reaction, esterification reaction, ionene-forming reaction, urethane-forming reaction and azlactone-phenol addition reaction.
Nakane Yoshinori and Ishidoya Masahiro, et al., Shikizai (Coloring Material), vol. 67, No. 12, pp. 766 to 774, 1994; Nakane Yoshinori and Ishidoya Masahiro, et al., Shikizai (Coloring Material), vol. 69, No. 11, pp. 735 to 742, 1996; JP-A No. 11-35675, describe a thermo-reversible cross-linked structure utilizing a vinyl ether group.
Further, there are examples as described below for obtaining a recycling property utilizing a thermo-reversible cross-linked structure with covalent bond.
JP-A No. 7-247364 describes a method for separating and recovering an oligomer and chemically-recycling this, utilizing a reversibly cross-linkable oligomer, and describes, as a method of cleaving a cross-linked portion, a means for irradiation with ultraviolet ray and a means for cleaving by heat utilizing a Diels-Alder reaction. However, for conducting a cleaving reaction uniformly utilizing light, it is difficult for a molded article itself to secure transparency against light and it is necessary to dilute and dissolve the molded article in an organic solvent before the reaction, and this procedure has extremely poor efficiency as compared with a usual material recycle of resin by heat melting. According to Example 3 of this publication, a cleaving reaction by heat occurs at 90° C. This cleaving temperature is equal to or lower than the glass transition temperature (90 to 105° C.) of a resin (polyacrylate) as a mother material, rather deteriorating heat resistance. When intending improvement in sufficient heat resistance at 100° C. or more, it is necessary that a cross-linked portion cleaving reaction occurs at temperatures at least over 120° C. Therefore, it is necessary to select a reversible cross-linking portion having suitable cleaving reaction temperature and apply this to a resin.
Japanese Patent Application National Publication (Laid-Open) No. 10-508655 attains a recycling property by introducing 2,5-dialkyl-substituted furan into a resin. Introduction of furan is performed by a dehydration reaction of a copolymer of carbon monoxide with an olefin, with a strong acid. However, in the case a biodegradable resin, it is polymerized by an easily-hydrolyzable functional group such as an ester bond and the like. Introduction of a furan ring by such means is very difficult since decomposition of a resin is caused by this. The cleaving temperature of a cross-linked portion and the thermal stability of a diene depend significantly on the polarity and concentration of a reaction field. In the case of biodegradable resin, it is not necessary to limit the Diels-Alder reaction to those using 2,5-dialkyl-substituted furan.
Further, examples utilizing a reversible reaction by an esterification reaction of an acid anhydride for improvement in heat resistance and improvement in a recycling property are described in JP-A No. 11-106578, and the like, and a means is shown in which a carboxylic anhydride is introduced into a vinyl polymerization compound and cross-linked with a linker having a hydroxyl group. However, a lot of biodegradable resins have in the main chain a hydrolysable bond in which a carboxylic acid acts as a catalyst, such as an ester bond and the like. When the esterification reaction of an acid anhydride is introduced into a biodegradable resin, a free carboxylic acid is generated when a cross-linked portion has been formed, and the hydrolysis speed of a biodegradable resin as a mother material becomes remarkably high in preservation of a resin before molding and in use of a molded article, consequently, the moisture resistance and durability of the resin lower more than the necessity and the resin cannot be practically used.
On the other hand, in the case of a carboxy-alkenyloxy type, when a compound having a bond cleaving temperature of 120° C. or more is used, a free carboxylic acid is not generated easily at practical temperatures of 100° C. or less. When a resin is previously dried sufficiently in molding, hydrolysis does not occur and its durability is not deteriorated. Further, a nitroso dimer type, urethane type and azlactone-hydroxyaryl type are also applicable.
There is also an example of introduction of a thermo-reversible cross-linked structure by an electrostatic bond into a biodegradable resin. First, as examples of electrostatic bond, there are JP-A No. 2000-281805 and, Yano Shinichi, Ionomer no Bussei to Kougyouteki Ouyou (Physical Property and Industrial Application of Ionomer); M. R. Tant et al., Ionomers (ISBN: 0-7514-0392-X).
As an example utilizing a thermo-reversible cross-linked structure by an electrostatic bond in a biodegradable resin, JP-A No. 2000-281805 discloses an ion-crosslinked film obtained by cross-linking a carboxyl group of polysaccharides such as carboxymethylcellulose having a carboxyl group, carboxyl group-containing starch and the like with a poly-valent metal ion, for the purpose of improving strength. However, in general, an electrostatic bond is inferior in bonding strength to a covalent bond, consequently, heat resistance cannot be desired to be sufficiently improved, though the viscosity and elastic modulus of a resin are improved remarkably.
As described above, there are a lot of trials for realizing the recycling property of a resin material by introducing a thermo-reversible cross-linked structure with covalent bond in the resin material, however, there is few example applying this in a biodegradable resin material. It is technologically difficult to introduce a thermo-reversible cross-linked structure with covalent bond into a biodegradable resin material, and is has been difficult to realize practical properties with a recyclable biodegradable resin material.